This is Info file ./gdb.info, produced by Makeinfo-1.55 from the input
file gdb.texinfo.

START-INFO-DIR-ENTRY
* Gdb: (gdb).                     The GNU debugger.
END-INFO-DIR-ENTRY
   This file documents the GNU debugger GDB.

   This is Edition 4.12, January 1994, of `Debugging with GDB: the GNU
Source-Level Debugger' for GDB Version 4.14.

   Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995 Free
Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.


File: gdb.info,  Node: Input/Output,  Next: Attach,  Prev: Working Directory,  Up: Running

Your program's input and output
===============================

   By default, the program you run under GDB does input and output to
the same terminal that GDB uses.  GDB switches the terminal to its own
terminal modes to interact with you, but it records the terminal modes
your program was using and switches back to them when you continue
running your program.

`info terminal'
     Displays information recorded by GDB about the terminal modes your
     program is using.

   You can redirect your program's input and/or output using shell
redirection with the `run' command.  For example,

     run > outfile

starts your program, diverting its output to the file `outfile'.

   Another way to specify where your program should do input and output
is with the `tty' command.  This command accepts a file name as
argument, and causes this file to be the default for future `run'
commands.  It also resets the controlling terminal for the child
process, for future `run' commands.  For example,

     tty /dev/ttyb

directs that processes started with subsequent `run' commands default
to do input and output on the terminal `/dev/ttyb' and have that as
their controlling terminal.

   An explicit redirection in `run' overrides the `tty' command's
effect on the input/output device, but not its effect on the controlling
terminal.

   When you use the `tty' command or redirect input in the `run'
command, only the input *for your program* is affected.  The input for
GDB still comes from your terminal.


File: gdb.info,  Node: Attach,  Next: Kill Process,  Prev: Input/Output,  Up: Running

Debugging an already-running process
====================================

`attach PROCESS-ID'
     This command attaches to a running process--one that was started
     outside GDB.  (`info files' shows your active targets.)  The
     command takes as argument a process ID.  The usual way to find out
     the process-id of a Unix process is with the `ps' utility, or with
     the `jobs -l' shell command.

     `attach' does not repeat if you press RET a second time after
     executing the command.

   To use `attach', your program must be running in an environment
which supports processes; for example, `attach' does not work for
programs on bare-board targets that lack an operating system.  You must
also have permission to send the process a signal.

   When using `attach', you should first use the `file' command to
specify the program running in the process and load its symbol table.
*Note Commands to Specify Files: Files.

   The first thing GDB does after arranging to debug the specified
process is to stop it.  You can examine and modify an attached process
with all the GDB commands that are ordinarily available when you start
processes with `run'.  You can insert breakpoints; you can step and
continue; you can modify storage.  If you would rather the process
continue running, you may use the `continue' command after attaching
GDB to the process.

`detach'
     When you have finished debugging the attached process, you can use
     the `detach' command to release it from GDB control.  Detaching
     the process continues its execution.  After the `detach' command,
     that process and GDB become completely independent once more, and
     you are ready to `attach' another process or start one with `run'.
     `detach' does not repeat if you press RET again after executing
     the command.

   If you exit GDB or use the `run' command while you have an attached
process, you kill that process.  By default, GDB asks for confirmation
if you try to do either of these things; you can control whether or not
you need to confirm by using the `set confirm' command (*note Optional
warnings and messages: Messages/Warnings.).


File: gdb.info,  Node: Kill Process,  Next: Process Information,  Prev: Attach,  Up: Running

Killing the child process
=========================

`kill'
     Kill the child process in which your program is running under GDB.

   This command is useful if you wish to debug a core dump instead of a
running process.  GDB ignores any core dump file while your program is
running.

   On some operating systems, a program cannot be executed outside GDB
while you have breakpoints set on it inside GDB.  You can use the
`kill' command in this situation to permit running your program outside
the debugger.

   The `kill' command is also useful if you wish to recompile and
relink your program, since on many systems it is impossible to modify an
executable file while it is running in a process.  In this case, when
you next type `run', GDB notices that the file has changed, and reads
the symbol table again (while trying to preserve your current
breakpoint settings).


File: gdb.info,  Node: Process Information,  Next: Threads,  Prev: Kill Process,  Up: Running

Additional process information
==============================

   Some operating systems provide a facility called `/proc' that can be
used to examine the image of a running process using file-system
subroutines.  If GDB is configured for an operating system with this
facility, the command `info proc' is available to report on several
kinds of information about the process running your program.

`info proc'
     Summarize available information about the process.

`info proc mappings'
     Report on the address ranges accessible in the program, with
     information on whether your program may read, write, or execute
     each range.

`info proc times'
     Starting time, user CPU time, and system CPU time for your program
     and its children.

`info proc id'
     Report on the process IDs related to your program: its own process
     ID, the ID of its parent, the process group ID, and the session ID.

`info proc status'
     General information on the state of the process.  If the process is
     stopped, this report includes the reason for stopping, and any
     signal received.

`info proc all'
     Show all the above information about the process.


File: gdb.info,  Node: Threads,  Next: Processes,  Prev: Process Information,  Up: Running

Debugging programs with multiple threads
========================================

   In some operating systems, a single program may have more than one
"thread" of execution.  The precise semantics of threads differ from
one operating system to another, but in general the threads of a single
program are akin to multiple processes--except that they share one
address space (that is, they can all examine and modify the same
variables).  On the other hand, each thread has its own registers and
execution stack, and perhaps private memory.

   GDB provides these facilities for debugging multi-thread programs:

   * automatic notification of new threads

   * `thread THREADNO', a command to switch among threads

   * `info threads', a command to inquire about existing threads

   * thread-specific breakpoints

     *Warning:* These facilities are not yet available on every GDB
     configuration where the operating system supports threads.  If
     your GDB does not support threads, these commands have no effect.
     For example, a system without thread support shows no output from
     `info threads', and always rejects the `thread' command, like this:

          (gdb) info threads
          (gdb) thread 1
          Thread ID 1 not known.  Use the "info threads" command to
          see the IDs of currently known threads.

   The GDB thread debugging facility allows you to observe all threads
while your program runs--but whenever GDB takes control, one thread in
particular is always the focus of debugging.  This thread is called the
"current thread".  Debugging commands show program information from the
perspective of the current thread.

   Whenever GDB detects a new thread in your program, it displays the
target system's identification for the thread with a message in the
form `[New SYSTAG]'.  SYSTAG is a thread identifier whose form varies
depending on the particular system.  For example, on LynxOS, you might
see

     [New process 35 thread 27]

when GDB notices a new thread.  In contrast, on an SGI system, the
SYSTAG is simply something like `process 368', with no further
qualifier.

   For debugging purposes, GDB associates its own thread number--always
a single integer--with each thread in your program.

`info threads'
     Display a summary of all threads currently in your program.  GDB
     displays for each thread (in this order):

       1. the thread number assigned by GDB

       2. the target system's thread identifier (SYSTAG)

       3. the current stack frame summary for that thread

     An asterisk `*' to the left of the GDB thread number indicates the
     current thread.

     For example,

     (gdb) info threads
       3 process 35 thread 27  0x34e5 in sigpause ()
       2 process 35 thread 23  0x34e5 in sigpause ()
     * 1 process 35 thread 13  main (argc=1, argv=0x7ffffff8)
         at threadtest.c:68

`thread THREADNO'
     Make thread number THREADNO the current thread.  The command
     argument THREADNO is the internal GDB thread number, as shown in
     the first field of the `info threads' display.  GDB responds by
     displaying the system identifier of the thread you selected, and
     its current stack frame summary:

          (gdb) thread 2
          [Switching to process 35 thread 23]
          0x34e5 in sigpause ()

     As with the `[New ...]' message, the form of the text after
     `Switching to' depends on your system's conventions for identifying
     threads.

   Whenever GDB stops your program, due to a breakpoint or a signal, it
automatically selects the thread where that breakpoint or signal
happened.  GDB alerts you to the context switch with a message of the
form `[Switching to SYSTAG]' to identify the thread.

   *Note Stopping and starting multi-thread programs: Thread Stops, for
more information about how GDB behaves when you stop and start programs
with multiple threads.

   *Note Setting watchpoints: Set Watchpoints, for information about
watchpoints in programs with multiple threads.


File: gdb.info,  Node: Processes,  Prev: Threads,  Up: Running

Debugging programs with multiple processes
==========================================

   GDB has no special support for debugging programs which create
additional processes using the `fork' function.  When a program forks,
GDB will continue to debug the parent process and the child process
will run unimpeded.  If you have set a breakpoint in any code which the
child then executes, the child will get a `SIGTRAP' signal which
(unless it catches the signal) will cause it to terminate.

   However, if you want to debug the child process there is a workaround
which isn't too painful.  Put a call to `sleep' in the code which the
child process executes after the fork.  It may be useful to sleep only
if a certain environment variable is set, or a certain file exists, so
that the delay need not occur when you don't want to run GDB on the
child.  While the child is sleeping, use the `ps' program to get its
process ID.  Then tell GDB (a new invocation of GDB if you are also
debugging the parent process) to attach to the child process (see *Note
Attach::).  From that point on you can debug the child process just
like any other process which you attached to.


File: gdb.info,  Node: Stopping,  Next: Stack,  Prev: Running,  Up: Top

Stopping and Continuing
***********************

   The principal purposes of using a debugger are so that you can stop
your program before it terminates; or so that, if your program runs into
trouble, you can investigate and find out why.

   Inside GDB, your program may stop for any of several reasons, such as
a signal, a breakpoint, or reaching a new line after a GDB command such
as `step'.  You may then examine and change variables, set new
breakpoints or remove old ones, and then continue execution.  Usually,
the messages shown by GDB provide ample explanation of the status of
your program--but you can also explicitly request this information at
any time.

`info program'
     Display information about the status of your program: whether it is
     running or not, what process it is, and why it stopped.

* Menu:


* Breakpoints::                 Breakpoints, watchpoints, and exceptions

* Continuing and Stepping::     Resuming execution

* Signals::                     Signals

* Thread Stops::		Stopping and starting multi-thread programs


File: gdb.info,  Node: Breakpoints,  Next: Continuing and Stepping,  Up: Stopping

Breakpoints, watchpoints, and exceptions
========================================

   A "breakpoint" makes your program stop whenever a certain point in
the program is reached.  For each breakpoint, you can add conditions to
control in finer detail whether your program stops.  You can set
breakpoints with the `break' command and its variants (*note Setting
breakpoints: Set Breaks.), to specify the place where your program
should stop by line number, function name or exact address in the
program.  In languages with exception handling (such as GNU C++), you
can also set breakpoints where an exception is raised (*note
Breakpoints and exceptions: Exception Handling.).

   A "watchpoint" is a special breakpoint that stops your program when
the value of an expression changes.  You must use a different command
to set watchpoints (*note Setting watchpoints: Set Watchpoints.), but
aside from that, you can manage a watchpoint like any other breakpoint:
you enable, disable, and delete both breakpoints and watchpoints using
the same commands.

   You can arrange to have values from your program displayed
automatically whenever GDB stops at a breakpoint.  *Note Automatic
display: Auto Display.

   GDB assigns a number to each breakpoint or watchpoint when you
create it; these numbers are successive integers starting with one.  In
many of the commands for controlling various features of breakpoints you
use the breakpoint number to say which breakpoint you want to change.
Each breakpoint may be "enabled" or "disabled"; if disabled, it has no
effect on your program until you enable it again.

* Menu:

* Set Breaks::                  Setting breakpoints
* Set Watchpoints::             Setting watchpoints

* Exception Handling::          Breakpoints and exceptions

* Delete Breaks::               Deleting breakpoints
* Disabling::                   Disabling breakpoints
* Conditions::                  Break conditions
* Break Commands::              Breakpoint command lists

* Breakpoint Menus::            Breakpoint menus

* Error in Breakpoints::        "Cannot insert breakpoints"


File: gdb.info,  Node: Set Breaks,  Next: Set Watchpoints,  Up: Breakpoints

Setting breakpoints
-------------------

   Breakpoints are set with the `break' command (abbreviated `b').  The
debugger convenience variable `$bpnum' records the number of the
beakpoint you've set most recently; see *Note Convenience variables:
Convenience Vars, for a discussion of what you can do with convenience
variables.

   You have several ways to say where the breakpoint should go.

`break FUNCTION'
     Set a breakpoint at entry to function FUNCTION.  When using source
     languages that permit overloading of symbols, such as C++,
     FUNCTION may refer to more than one possible place to break.
     *Note Breakpoint menus: Breakpoint Menus, for a discussion of that
     situation.

`break +OFFSET'
`break -OFFSET'
     Set a breakpoint some number of lines forward or back from the
     position at which execution stopped in the currently selected
     frame.

`break LINENUM'
     Set a breakpoint at line LINENUM in the current source file.  That
     file is the last file whose source text was printed.  This
     breakpoint stops your program just before it executes any of the
     code on that line.

`break FILENAME:LINENUM'
     Set a breakpoint at line LINENUM in source file FILENAME.

`break FILENAME:FUNCTION'
     Set a breakpoint at entry to function FUNCTION found in file
     FILENAME.  Specifying a file name as well as a function name is
     superfluous except when multiple files contain similarly named
     functions.

`break *ADDRESS'
     Set a breakpoint at address ADDRESS.  You can use this to set
     breakpoints in parts of your program which do not have debugging
     information or source files.

`break'
     When called without any arguments, `break' sets a breakpoint at
     the next instruction to be executed in the selected stack frame
     (*note Examining the Stack: Stack.).  In any selected frame but the
     innermost, this makes your program stop as soon as control returns
     to that frame.  This is similar to the effect of a `finish'
     command in the frame inside the selected frame--except that
     `finish' does not leave an active breakpoint.  If you use `break'
     without an argument in the innermost frame, GDB stops the next
     time it reaches the current location; this may be useful inside
     loops.

     GDB normally ignores breakpoints when it resumes execution, until
     at least one instruction has been executed.  If it did not do
     this, you would be unable to proceed past a breakpoint without
     first disabling the breakpoint.  This rule applies whether or not
     the breakpoint already existed when your program stopped.

`break ... if COND'
     Set a breakpoint with condition COND; evaluate the expression COND
     each time the breakpoint is reached, and stop only if the value is
     nonzero--that is, if COND evaluates as true.  `...' stands for one
     of the possible arguments described above (or no argument)
     specifying where to break.  *Note Break conditions: Conditions,
     for more information on breakpoint conditions.

`tbreak ARGS'
     Set a breakpoint enabled only for one stop.  ARGS are the same as
     for the `break' command, and the breakpoint is set in the same
     way, but the breakpoint is automatically deleted after the first
     time your program stops there.  *Note Disabling breakpoints:
     Disabling.

`rbreak REGEX'
     Set breakpoints on all functions matching the regular expression
     REGEX.  This command sets an unconditional breakpoint on all
     matches, printing a list of all breakpoints it set.  Once these
     breakpoints are set, they are treated just like the breakpoints
     set with the `break' command.  You can delete them, disable them,
     or make them conditional the same way as any other breakpoint.

     When debugging C++ programs, `rbreak' is useful for setting
     breakpoints on overloaded functions that are not members of any
     special classes.

`info breakpoints [N]'
`info break [N]'
`info watchpoints [N]'
     Print a table of all breakpoints and watchpoints set and not
     deleted, with the following columns for each breakpoint:

    *Breakpoint Numbers*
    *Type*
          Breakpoint or watchpoint.

    *Disposition*
          Whether the breakpoint is marked to be disabled or deleted
          when hit.

    *Enabled or Disabled*
          Enabled breakpoints are marked with `y'.  `n' marks
          breakpoints that are not enabled.

    *Address*
          Where the breakpoint is in your program, as a memory address

    *What*
          Where the breakpoint is in the source for your program, as a
          file and line number.

     If a breakpoint is conditional, `info break' shows the condition on
     the line following the affected breakpoint; breakpoint commands,
     if any, are listed after that.

     `info break' with a breakpoint number N as argument lists only
     that breakpoint.  The convenience variable `$_' and the default
     examining-address for the `x' command are set to the address of
     the last breakpoint listed (*note Examining memory: Memory.).

   GDB allows you to set any number of breakpoints at the same place in
your program.  There is nothing silly or meaningless about this.  When
the breakpoints are conditional, this is even useful (*note Break
conditions: Conditions.).

   GDB itself sometimes sets breakpoints in your program for special
purposes, such as proper handling of `longjmp' (in C programs).  These
internal breakpoints are assigned negative numbers, starting with `-1';
`info breakpoints' does not display them.

   You can see these breakpoints with the GDB maintenance command
`maint info breakpoints'.

`maint info breakpoints'
     Using the same format as `info breakpoints', display both the
     breakpoints you've set explicitly, and those GDB is using for
     internal purposes.  Internal breakpoints are shown with negative
     breakpoint numbers.  The type column identifies what kind of
     breakpoint is shown:

    `breakpoint'
          Normal, explicitly set breakpoint.

    `watchpoint'
          Normal, explicitly set watchpoint.

    `longjmp'
          Internal breakpoint, used to handle correctly stepping through
          `longjmp' calls.

    `longjmp resume'
          Internal breakpoint at the target of a `longjmp'.

    `until'
          Temporary internal breakpoint used by the GDB `until' command.

    `finish'
          Temporary internal breakpoint used by the GDB `finish'
          command.


File: gdb.info,  Node: Set Watchpoints,  Next: Exception Handling,  Prev: Set Breaks,  Up: Breakpoints

Setting watchpoints
-------------------

   You can use a watchpoint to stop execution whenever the value of an
expression changes, without having to predict a particular place where
this may happen.

   Watchpoints currently execute two orders of magnitude more slowly
than other breakpoints, but this can be well worth it to catch errors
where you have no clue what part of your program is the culprit.

   Some processors provide special hardware to support watchpoint
evaluation; GDB will use such hardware if it is available, and if the
support code has been added for that configuration.

`watch EXPR'
     Set a watchpoint for an expression.

`info watchpoints'
     This command prints a list of watchpoints and breakpoints; it is
     the same as `info break'.

     *Warning:* in multi-thread programs, watchpoints have only limited
     usefulness.  With the current watchpoint implementation, GDB can
     only watch the value of an expression *in a single thread*.  If
     you are confident that the expression can only change due to the
     current thread's activity (and if you are also confident that no
     other thread can become current), then you can use watchpoints as
     usual.  However, GDB may not notice when a non-current thread's
     activity changes the expression.


File: gdb.info,  Node: Exception Handling,  Next: Delete Breaks,  Prev: Set Watchpoints,  Up: Breakpoints

Breakpoints and exceptions
--------------------------

   Some languages, such as GNU C++, implement exception handling.  You
can use GDB to examine what caused your program to raise an exception,
and to list the exceptions your program is prepared to handle at a
given point in time.

`catch EXCEPTIONS'
     You can set breakpoints at active exception handlers by using the
     `catch' command.  EXCEPTIONS is a list of names of exceptions to
     catch.

   You can use `info catch' to list active exception handlers.  *Note
Information about a frame: Frame Info.

   There are currently some limitations to exception handling in GDB:

   * If you call a function interactively, GDB normally returns control
     to you when the function has finished executing.  If the call
     raises an exception, however, the call may bypass the mechanism
     that returns control to you and cause your program to simply
     continue running until it hits a breakpoint, catches a signal that
     GDB is listening for, or exits.

   * You cannot raise an exception interactively.

   * You cannot install an exception handler interactively.

   Sometimes `catch' is not the best way to debug exception handling:
if you need to know exactly where an exception is raised, it is better
to stop *before* the exception handler is called, since that way you
can see the stack before any unwinding takes place.  If you set a
breakpoint in an exception handler instead, it may not be easy to find
out where the exception was raised.

   To stop just before an exception handler is called, you need some
knowledge of the implementation.  In the case of GNU C++, exceptions are
raised by calling a library function named `__raise_exception' which
has the following ANSI C interface:

         /* ADDR is where the exception identifier is stored.
            ID is the exception identifier.  */
         void __raise_exception (void **ADDR, void *ID);

To make the debugger catch all exceptions before any stack unwinding
takes place, set a breakpoint on `__raise_exception' (*note
Breakpoints; watchpoints; and exceptions: Breakpoints.).

   With a conditional breakpoint (*note Break conditions: Conditions.)
that depends on the value of ID, you can stop your program when a
specific exception is raised.  You can use multiple conditional
breakpoints to stop your program when any of a number of exceptions are
raised.


File: gdb.info,  Node: Delete Breaks,  Next: Disabling,  Prev: Exception Handling,  Up: Breakpoints

Deleting breakpoints
--------------------

   It is often necessary to eliminate a breakpoint or watchpoint once it
has done its job and you no longer want your program to stop there.
This is called "deleting" the breakpoint.  A breakpoint that has been
deleted no longer exists; it is forgotten.

   With the `clear' command you can delete breakpoints according to
where they are in your program.  With the `delete' command you can
delete individual breakpoints or watchpoints by specifying their
breakpoint numbers.

   It is not necessary to delete a breakpoint to proceed past it.  GDB
automatically ignores breakpoints on the first instruction to be
executed when you continue execution without changing the execution
address.

`clear'
     Delete any breakpoints at the next instruction to be executed in
     the selected stack frame (*note Selecting a frame: Selection.).
     When the innermost frame is selected, this is a good way to delete
     a breakpoint where your program just stopped.

`clear FUNCTION'
`clear FILENAME:FUNCTION'
     Delete any breakpoints set at entry to the function FUNCTION.

`clear LINENUM'
`clear FILENAME:LINENUM'
     Delete any breakpoints set at or within the code of the specified
     line.

`delete [breakpoints] [BNUMS...]'
     Delete the breakpoints or watchpoints of the numbers specified as
     arguments.  If no argument is specified, delete all breakpoints
     (GDB asks confirmation, unless you have `set confirm off').  You
     can abbreviate this command as `d'.


File: gdb.info,  Node: Disabling,  Next: Conditions,  Prev: Delete Breaks,  Up: Breakpoints

Disabling breakpoints
---------------------

   Rather than deleting a breakpoint or watchpoint, you might prefer to
"disable" it.  This makes the breakpoint inoperative as if it had been
deleted, but remembers the information on the breakpoint so that you
can "enable" it again later.

   You disable and enable breakpoints and watchpoints with the `enable'
and `disable' commands, optionally specifying one or more breakpoint
numbers as arguments.  Use `info break' or `info watch' to print a list
of breakpoints or watchpoints if you do not know which numbers to use.

   A breakpoint or watchpoint can have any of four different states of
enablement:

   * Enabled.  The breakpoint stops your program.  A breakpoint set
     with the `break' command starts out in this state.

   * Disabled.  The breakpoint has no effect on your program.

   * Enabled once.  The breakpoint stops your program, but then becomes
     disabled.  A breakpoint set with the `tbreak' command starts out in
     this state.

   * Enabled for deletion.  The breakpoint stops your program, but
     immediately after it does so it is deleted permanently.

   You can use the following commands to enable or disable breakpoints
and watchpoints:

`disable [breakpoints] [BNUMS...]'
     Disable the specified breakpoints--or all breakpoints, if none are
     listed.  A disabled breakpoint has no effect but is not forgotten.
     All options such as ignore-counts, conditions and commands are
     remembered in case the breakpoint is enabled again later.  You may
     abbreviate `disable' as `dis'.

`enable [breakpoints] [BNUMS...]'
     Enable the specified breakpoints (or all defined breakpoints).
     They become effective once again in stopping your program.

`enable [breakpoints] once BNUMS...'
     Enable the specified breakpoints temporarily.  GDB disables any of
     these breakpoints immediately after stopping your program.

`enable [breakpoints] delete BNUMS...'
     Enable the specified breakpoints to work once, then die.  GDB
     deletes any of these breakpoints as soon as your program stops
     there.

   Save for a breakpoint set with `tbreak' (*note Setting breakpoints:
Set Breaks.), breakpoints that you set are initially enabled;
subsequently, they become disabled or enabled only when you use one of
the commands above.  (The command `until' can set and delete a
breakpoint of its own, but it does not change the state of your other
breakpoints; see *Note Continuing and stepping: Continuing and
Stepping.)


File: gdb.info,  Node: Conditions,  Next: Break Commands,  Prev: Disabling,  Up: Breakpoints

Break conditions
----------------

   The simplest sort of breakpoint breaks every time your program
reaches a specified place.  You can also specify a "condition" for a
breakpoint.  A condition is just a Boolean expression in your
programming language (*note Expressions: Expressions.).  A breakpoint
with a condition evaluates the expression each time your program
reaches it, and your program stops only if the condition is *true*.

   This is the converse of using assertions for program validation; in
that situation, you want to stop when the assertion is violated--that
is, when the condition is false.  In C, if you want to test an
assertion expressed by the condition ASSERT, you should set the
condition `! ASSERT' on the appropriate breakpoint.

   Conditions are also accepted for watchpoints; you may not need them,
since a watchpoint is inspecting the value of an expression anyhow--but
it might be simpler, say, to just set a watchpoint on a variable name,
and specify a condition that tests whether the new value is an
interesting one.

   Break conditions can have side effects, and may even call functions
in your program.  This can be useful, for example, to activate functions
that log program progress, or to use your own print functions to format
special data structures. The effects are completely predictable unless
there is another enabled breakpoint at the same address.  (In that
case, GDB might see the other breakpoint first and stop your program
without checking the condition of this one.)  Note that breakpoint
commands are usually more convenient and flexible for the purpose of
performing side effects when a breakpoint is reached (*note Breakpoint
command lists: Break Commands.).

   Break conditions can be specified when a breakpoint is set, by using
`if' in the arguments to the `break' command.  *Note Setting
breakpoints: Set Breaks.  They can also be changed at any time with the
`condition' command.  The `watch' command does not recognize the `if'
keyword; `condition' is the only way to impose a further condition on a
watchpoint.

`condition BNUM EXPRESSION'
     Specify EXPRESSION as the break condition for breakpoint or
     watchpoint number BNUM.  After you set a condition, breakpoint
     BNUM stops your program only if the value of EXPRESSION is true
     (nonzero, in C).  When you use `condition', GDB checks EXPRESSION
     immediately for syntactic correctness, and to determine whether
     symbols in it have referents in the context of your breakpoint.
     G{No Value For "DBN"} does not actually evaluate EXPRESSION at the
     time the `condition' command is given, however.  *Note
     Expressions: Expressions.

`condition BNUM'
     Remove the condition from breakpoint number BNUM.  It becomes an
     ordinary unconditional breakpoint.

   A special case of a breakpoint condition is to stop only when the
breakpoint has been reached a certain number of times.  This is so
useful that there is a special way to do it, using the "ignore count"
of the breakpoint.  Every breakpoint has an ignore count, which is an
integer.  Most of the time, the ignore count is zero, and therefore has
no effect.  But if your program reaches a breakpoint whose ignore count
is positive, then instead of stopping, it just decrements the ignore
count by one and continues.  As a result, if the ignore count value is
N, the breakpoint does not stop the next N times your program reaches
it.

`ignore BNUM COUNT'
     Set the ignore count of breakpoint number BNUM to COUNT.  The next
     COUNT times the breakpoint is reached, your program's execution
     does not stop; other than to decrement the ignore count, GDB takes
     no action.

     To make the breakpoint stop the next time it is reached, specify a
     count of zero.

     When you use `continue' to resume execution of your program from a
     breakpoint, you can specify an ignore count directly as an
     argument to `continue', rather than using `ignore'.  *Note
     Continuing and stepping: Continuing and Stepping.

     If a breakpoint has a positive ignore count and a condition, the
     condition is not checked.  Once the ignore count reaches zero, GDB
     resumes checking the condition.

     You could achieve the effect of the ignore count with a condition
     such as `$foo-- <= 0' using a debugger convenience variable that
     is decremented each time.  *Note Convenience variables:
     Convenience Vars.


File: gdb.info,  Node: Break Commands,  Next: Breakpoint Menus,  Prev: Conditions,  Up: Breakpoints

Breakpoint command lists
------------------------

   You can give any breakpoint (or watchpoint) a series of commands to
execute when your program stops due to that breakpoint.  For example,
you might want to print the values of certain expressions, or enable
other breakpoints.

`commands [BNUM]'
`... COMMAND-LIST ...'
`end'
     Specify a list of commands for breakpoint number BNUM.  The
     commands themselves appear on the following lines.  Type a line
     containing just `end' to terminate the commands.

     To remove all commands from a breakpoint, type `commands' and
     follow it immediately with `end'; that is, give no commands.

     With no BNUM argument, `commands' refers to the last breakpoint or
     watchpoint set (not to the breakpoint most recently encountered).

   Pressing RET as a means of repeating the last GDB command is
disabled within a COMMAND-LIST.

   You can use breakpoint commands to start your program up again.
Simply use the `continue' command, or `step', or any other command that
resumes execution.

   Any other commands in the command list, after a command that resumes
execution, are ignored.  This is because any time you resume execution
(even with a simple `next' or `step'), you may encounter another
breakpoint--which could have its own command list, leading to
ambiguities about which list to execute.

   If the first command you specify in a command list is `silent', the
usual message about stopping at a breakpoint is not printed.  This may
be desirable for breakpoints that are to print a specific message and
then continue.  If none of the remaining commands print anything, you
see no sign that the breakpoint was reached.  `silent' is meaningful
only at the beginning of a breakpoint command list.

   The commands `echo', `output', and `printf' allow you to print
precisely controlled output, and are often useful in silent
breakpoints.  *Note Commands for controlled output: Output.

   For example, here is how you could use breakpoint commands to print
the value of `x' at entry to `foo' whenever `x' is positive.

     break foo if x>0
     commands
     silent
     printf "x is %d\n",x
     cont
     end

   One application for breakpoint commands is to compensate for one bug
so you can test for another.  Put a breakpoint just after the erroneous
line of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them.  End with the `continue' command so
that your program does not stop, and start with the `silent' command so
that no output is produced.  Here is an example:

     break 403
     commands
     silent
     set x = y + 4
     cont
     end


File: gdb.info,  Node: Breakpoint Menus,  Next: Error in Breakpoints,  Prev: Break Commands,  Up: Breakpoints

Breakpoint menus
----------------

   Some programming languages (notably C++) permit a single function
name to be defined several times, for application in different contexts.
This is called "overloading".  When a function name is overloaded,
`break FUNCTION' is not enough to tell GDB where you want a breakpoint.
If you realize this is a problem, you can use something like `break
FUNCTION(TYPES)' to specify which particular version of the function
you want.  Otherwise, GDB offers you a menu of numbered choices for
different possible breakpoints, and waits for your selection with the
prompt `>'.  The first two options are always `[0] cancel' and `[1]
all'.  Typing `1' sets a breakpoint at each definition of FUNCTION, and
typing `0' aborts the `break' command without setting any new
breakpoints.

   For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol `String::after'.  We choose three
particular definitions of that function name:

     (gdb) b String::after
     [0] cancel
     [1] all
     [2] file:String.cc; line number:867
     [3] file:String.cc; line number:860
     [4] file:String.cc; line number:875
     [5] file:String.cc; line number:853
     [6] file:String.cc; line number:846
     [7] file:String.cc; line number:735
     > 2 4 6
     Breakpoint 1 at 0xb26c: file String.cc, line 867.
     Breakpoint 2 at 0xb344: file String.cc, line 875.
     Breakpoint 3 at 0xafcc: file String.cc, line 846.
     Multiple breakpoints were set.
     Use the "delete" command to delete unwanted
      breakpoints.
     (gdb)


File: gdb.info,  Node: Error in Breakpoints,  Prev: Breakpoint Menus,  Up: Breakpoints

"Cannot insert breakpoints"
---------------------------

   Under some operating systems, breakpoints cannot be used in a
program if any other process is running that program.  In this
situation, attempting to run or continue a program with a breakpoint
causes GDB to stop the other process.

   When this happens, you have three ways to proceed:

  1. Remove or disable the breakpoints, then continue.

  2. Suspend GDB, and copy the file containing your program to a new
     name.  Resume GDB and use the `exec-file' command to specify that
     GDB should run your program under that name.  Then start your
     program again.

  3. Relink your program so that the text segment is nonsharable, using
     the linker option `-N'.  The operating system limitation may not
     apply to nonsharable executables.


File: gdb.info,  Node: Continuing and Stepping,  Next: Signals,  Prev: Breakpoints,  Up: Stopping

Continuing and stepping
=======================

   "Continuing" means resuming program execution until your program
completes normally.  In contrast, "stepping" means executing just one
more "step" of your program, where "step" may mean either one line of
source code, or one machine instruction (depending on what particular
command you use).  Either when continuing or when stepping, your
program may stop even sooner, due to a breakpoint or a signal.  (If due
to a signal, you may want to use `handle', or use `signal 0' to resume
execution.  *Note Signals: Signals.)

`continue [IGNORE-COUNT]'
`c [IGNORE-COUNT]'
`fg [IGNORE-COUNT]'
     Resume program execution, at the address where your program last
     stopped; any breakpoints set at that address are bypassed.  The
     optional argument IGNORE-COUNT allows you to specify a further
     number of times to ignore a breakpoint at this location; its
     effect is like that of `ignore' (*note Break conditions:
     Conditions.).

     The argument IGNORE-COUNT is meaningful only when your program
     stopped due to a breakpoint.  At other times, the argument to
     `continue' is ignored.

     The synonyms `c' and `fg' are provided purely for convenience, and
     have exactly the same behavior as `continue'.

   To resume execution at a different place, you can use `return'
(*note Returning from a function: Returning.) to go back to the calling
function; or `jump' (*note Continuing at a different address: Jumping.)
to go to an arbitrary location in your program.

   A typical technique for using stepping is to set a breakpoint (*note
Breakpoints; watchpoints; and exceptions: Breakpoints.) at the
beginning of the function or the section of your program where a
problem is believed to lie, run your program until it stops at that
breakpoint, and then step through the suspect area, examining the
variables that are interesting, until you see the problem happen.

`step'
     Continue running your program until control reaches a different
     source line, then stop it and return control to GDB.  This command
     is abbreviated `s'.

          *Warning:* If you use the `step' command while control is
          within a function that was compiled without debugging
          information, execution proceeds until control reaches a
          function that does have debugging information.  Likewise, it
          will not step into a function which is compiled without
          debugging information.  To step through functions without
          debugging information, use the `stepi' command, described
          below.

`step COUNT'
     Continue running as in `step', but do so COUNT times.  If a
     breakpoint is reached, or a signal not related to stepping occurs
     before COUNT steps, stepping stops right away.

`next [COUNT]'
     Continue to the next source line in the current (innermost) stack
     frame.  Similar to `step', but any function calls appearing within
     the line of code are executed without stopping.  Execution stops
     when control reaches a different line of code at the stack level
     which was executing when the `next' command was given.  This
     command is abbreviated `n'.

     An argument COUNT is a repeat count, as for `step'.

     `next' within a function that lacks debugging information acts like
     `step', but any function calls appearing within the code of the
     function are executed without stopping.

`finish'
     Continue running until just after function in the selected stack
     frame returns.  Print the returned value (if any).

     Contrast this with the `return' command (*note Returning from a
     function: Returning.).

`until'
`u'
     Continue running until a source line past the current line, in the
     current stack frame, is reached.  This command is used to avoid
     single stepping through a loop more than once.  It is like the
     `next' command, except that when `until' encounters a jump, it
     automatically continues execution until the program counter is
     greater than the address of the jump.

     This means that when you reach the end of a loop after single
     stepping though it, `until' makes your program continue execution
     until it exits the loop.  In contrast, a `next' command at the end
     of a loop simply steps back to the beginning of the loop, which
     forces you to step through the next iteration.

     `until' always stops your program if it attempts to exit the
     current stack frame.

     `until' may produce somewhat counterintuitive results if the order
     of machine code does not match the order of the source lines.  For
     example, in the following excerpt from a debugging session, the `f'
     (`frame') command shows that execution is stopped at line `206';
     yet when we use `until', we get to line `195':

          (gdb) f
          #0  main (argc=4, argv=0xf7fffae8) at m4.c:206
          206                 expand_input();
          (gdb) until
          195             for ( ; argc > 0; NEXTARG) {

     This happened because, for execution efficiency, the compiler had
     generated code for the loop closure test at the end, rather than
     the start, of the loop--even though the test in a C `for'-loop is
     written before the body of the loop.  The `until' command appeared
     to step back to the beginning of the loop when it advanced to this
     expression; however, it has not really gone to an earlier
     statement--not in terms of the actual machine code.

     `until' with no argument works by means of single instruction
     stepping, and hence is slower than `until' with an argument.

`until LOCATION'
`u LOCATION'
     Continue running your program until either the specified location
     is reached, or the current stack frame returns.  LOCATION is any of
     the forms of argument acceptable to `break' (*note Setting
     breakpoints: Set Breaks.).  This form of the command uses
     breakpoints, and hence is quicker than `until' without an argument.

`stepi'
`si'
     Execute one machine instruction, then stop and return to the
     debugger.

     It is often useful to do `display/i $pc' when stepping by machine
     instructions.  This makes GDB automatically display the next
     instruction to be executed, each time your program stops.  *Note
     Automatic display: Auto Display.

     An argument is a repeat count, as in `step'.

`nexti'
`ni'
     Execute one machine instruction, but if it is a function call,
     proceed until the function returns.

     An argument is a repeat count, as in `next'.


File: gdb.info,  Node: Signals,  Next: Thread Stops,  Prev: Continuing and Stepping,  Up: Stopping

Signals
=======

   A signal is an asynchronous event that can happen in a program.  The
operating system defines the possible kinds of signals, and gives each
kind a name and a number.  For example, in Unix `SIGINT' is the signal
a program gets when you type an interrupt (often `C-c'); `SIGSEGV' is
the signal a program gets from referencing a place in memory far away
from all the areas in use; `SIGALRM' occurs when the alarm clock timer
goes off (which happens only if your program has requested an alarm).

   Some signals, including `SIGALRM', are a normal part of the
functioning of your program.  Others, such as `SIGSEGV', indicate
errors; these signals are "fatal" (kill your program immediately) if the
program has not specified in advance some other way to handle the
signal.  `SIGINT' does not indicate an error in your program, but it is
normally fatal so it can carry out the purpose of the interrupt: to
kill the program.

   GDB has the ability to detect any occurrence of a signal in your
program.  You can tell GDB in advance what to do for each kind of
signal.

   Normally, GDB is set up to ignore non-erroneous signals like
`SIGALRM' (so as not to interfere with their role in the functioning of
your program) but to stop your program immediately whenever an error
signal happens.  You can change these settings with the `handle'
command.

`info signals'
     Print a table of all the kinds of signals and how GDB has been
     told to handle each one.  You can use this to see the signal
     numbers of all the defined types of signals.

`handle SIGNAL KEYWORDS...'
     Change the way GDB handles signal SIGNAL.  SIGNAL can be the
     number of a signal or its name (with or without the `SIG' at the
     beginning).  The KEYWORDS say what change to make.

   The keywords allowed by the `handle' command can be abbreviated.
Their full names are:

`nostop'
     GDB should not stop your program when this signal happens.  It may
     still print a message telling you that the signal has come in.

`stop'
     GDB should stop your program when this signal happens.  This
     implies the `print' keyword as well.

`print'
     GDB should print a message when this signal happens.

`noprint'
     GDB should not mention the occurrence of the signal at all.  This
     implies the `nostop' keyword as well.

`pass'
     GDB should allow your program to see this signal; your program can
     handle the signal, or else it may terminate if the signal is fatal
     and not handled.

`nopass'
     GDB should not allow your program to see this signal.

   When a signal stops your program, the signal is not visible until you
continue.  Your program sees the signal then, if `pass' is in effect
for the signal in question *at that time*.  In other words, after GDB
reports a signal, you can use the `handle' command with `pass' or
`nopass' to control whether your program sees that signal when you
continue.

   You can also use the `signal' command to prevent your program from
seeing a signal, or cause it to see a signal it normally would not see,
or to give it any signal at any time.  For example, if your program
stopped due to some sort of memory reference error, you might store
correct values into the erroneous variables and continue, hoping to see
more execution; but your program would probably terminate immediately as
a result of the fatal signal once it saw the signal.  To prevent this,
you can continue with `signal 0'.  *Note Giving your program a signal:
Signaling.

